Unfortunately I have to make your life a living hell… It’s not just Germans, if I were to guess it’s most Europeans. Or at least everywhere I’ve been (Denmark, Czech Republic, Poland, Hungary) it’s been a thing.
If I were to have to make an educated guess it’s because houses in Europe aren’t necessarily new to put it mildly. Some have been around for centuries or at least since the commie years with smaller upgrades and simply in some cases to avoid mold you gotta let the humor air out.
Plus if I’m thinking about myself and why I do it… it’s because I like the fresh air even if it’s cold or sometimes especially if it’s cold! Helps you sleep better and such